Phase diagram of water - Columbia University

1 Phase diagram of waterNote: for H2O melting point decreases with increasing pressure, for CO2melting point increases with increasing ~ 70% of the earth s surfaceLife on earth depends on waterWater is a universal solvent Easily polluted; hard to , water is an anomaly water is the most remarkable substance. However, water is often perceived to be pretty ordinary. We wash in water , fish in water , swim in water , drink water and cook with water We are about two-thirds water and require water to live. Life as we know it could not have evolved without waterand dies without it. Droughts cause famines and floods cause death and disease. Because of its clear importance, water is the most studied material on Earth. It comes as a surprise, therefore, to find that it is so poorly understood, not only by people in general, but also by scientists working with it everyday.

2 Chapin , Professor of Applied ScienceSouth Bank UniversityLondon SE1 0AA the long term existence of our watery planet as a place hospitable for the evolution of life involves a considerable amount of good luck. Today, more than a billion people have to walk over a mile every day for their drinking water , 80 percent of all diseases result directly from drinking polluted water , and many believe future wars will be fought over fresh water , water : The Drop of Life, PBS, 2000 Why is water unique?Shape of water moleculeLewis dot filling modelO-H bonds are polarized because of the difference in electronegativity between the O and H atomsHydrogen bondsThis unequal electron distribution results in strong non-bonding interactions between water molecules - hydrogen bondsHydrogen bonds in liquid waterConsequences of hydrogen bonding in waterIce floats because hydrogen bonds hold water molecules further apart in a solid than in a liquid - density of ice is less than density of waterDensity of ice at 0oC - g/mlDensity of water at 0oC - g/mlliquid watersolid has a high boiling point and high freezing water were normal it would be a gas at room , NH3, freeze at lower temperatures than H2 OBoiling points vs molecular massWater has a high specific heat index.

3 It takes much more heat to raise the temperature of a volume of water than the same volume of is why water is used as a coolant. This property of water is a major determinant of global climates and rates of global climate change- changes in temperatures are gradual- lake effectsWater has a high surface tensionwater molecules are cohesive - 'stick' to one another and to other polar moleculesNamesurface tension (dynes/cm at 20oC)Water73 Methanol22 Ethanol22 Ether17insect on waterThe surface tension makes air- water boundaries distinctive microhabitats. Universal SolventWater being polar can dissolve ionic and polar compoundsPolar compounds in waterSince water is a universal solvent, water is easily readily dissolves compounds ( heavy metal ions, pesticides), and hard to is critical for the three-dimensional structure and activity of proteins, nucleic acids.

4 Hydrophobic effect , or the exclusion of non-polar compounds is another unique property of water caused by the hydrogen bonds. The hydrophobic effect is particularly important in the formation of cell membranes - water "squeezes" non-polar molecules togetherInteresting water ChangesMolecules in a liquid are in constant motion; some moving faster, others molecules with enough kinetic energy escape from the liquid surface, : When molecules in the gas Phase collide with the liquid surface, they loose energy and return to the some point the rate of vaporization and the rate of condensation become equal and the system is at equilibrium. The partial pressure of the vapor above the liquid established at equilibrium is called the equilibrium vapor pressure or just vapor Point- the temperature at which the vapor pressure of the liquid equals the atmospheric boiling point - temperature at which the vapor pressure equals 1 atm.

5 Melting point- temperature at which a substance turns from solid to three Phase changes can be brought about by changes in temperature or pressure:Fusion or meltingsolid --> liquidVaporizationliquid --> gasSublimationsolid --> gassolidliquidgasSolutionsSolutions - homogenous mixture of two or more componentsLiquid solutionsSolute(gas, liquid or solid) dissolved in a liquid SolventIf solute and solvent are liquids, and the resulting solution ishomogenous, the two are said to be miscible. Mass percentage =mass of componenttotal mass of mixtureX 100%molality (m) =moles of solutekilograms of solvent(units mol/kg)Concentrations of Solutionsmolarity (M) =moles of soluteliter of solution(units mol/l)total number of moles in mixtureMole fraction =moles of componentFor a solution of two components A & BXA=nA+ nBnAXB= 1 - XAEffect of Temperature and Pressure on SolubilityDissolution is often accompanied by a change in solid and liquid solutes, temperature can effect solubility.

6 Pressure has little or no gas solutes - temperature and pressure can effect solubilityNH4NO3(s) NH4NO3(aq) O2(g) O2(aq) pO2= KHXO2 Henry s Lawwhere is the partial pressure of the O2above the solutionis the mole fraction of the gas solute in solutionKH- Henry s constant (units of pressure, as expressed above)pO2XO2pO2= KH[O2]units of KH: L-atm/mol Henry s law determines the amount of dissolved oxygen in water at a given 25oC, the solubility of O2is of waterVerify this if at 25oC KHis , and pO2is atmThermal pollution effects the mole fraction of O2in water .